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East African mountain lakes:
Archives for past climate change and glacier dynamics
Hilde Eggermont
The sedimentary record of Africa’s
alpine lakes contain valuable and
largely untapped records of past climate change and glacier dynamics.
Our findings suggest that the influence of late 19th century reductions
in precipitation in triggering Rwenzori glacier recession is weaker than
previously thought
Tropical glaciers are enormously important to our understanding of past,
present, and future climate change, yet
lack of continuous quantitative records
of alpine glacial extent in tropical East
Africa prior to the 20th century has left
the precise timing and drivers of recent
glacial advances and recession in the
region equivocal. Sediments accumulating on the bottom of alpine glacial
lakes chronicle the history of central
African climate and environmental dynamics, and can thus provide an historical perspective to understand the climatic controls on glacier mass balance,
the relative impacts of human-induced
global warming versus natural climate
variability, and the long-term tropical
mountain ecosystem and glacier stability in East Africa.
The main goal of our research program (2005 - present) is to investigate
century-scale variability in rainfall and
temperature and its impact on alpine
glaciation and ecosystems in central
equatorial Africa during the mid- to late
Holocene through multi-proxy analysis
of sediment records from East African
mountain lakes. Our current efforts are
to:
• Actively monitor the physical and
chemical limnology of a large set of
East African mountain lakes, and
collect associated climate data
• Calibrate sedimentological, geoche14
Lake Batoda located at 4017 m in the Rwenzori Mountains (Uganda), surrounded by tree groundsel.
© Hilde Eggermont
mical and biological climate-proxy
indicators through analysis of their
variation in the surface sediments of
lakes and pools located along a large
altitudinal (temperature) gradient
• Analyse long sediment cores from
the most promising sites to construct
a regional picture of African paleoclimate history over the past 500010,000 years.
• Document the poorly known biodiversity of aquatic algae, insects and
micro-crustacea in the unique setting of tropical high-elevation lakes;
and analyse their long-term response
to climate change and glacier dynamics.
To this end, we maintain an active field
and laboratory research programme in
three East African mountain regions:
the Rwenzori Mountains (Uganda-DR
Congo; Eggermont et al. 2009b), Mount
Kenya (Kenya) and the Bale Mountains (Ethiopia); the former two still
have snow-capped summits. In January 2008, we recovered long cores from
seven lakes in the Rwenzori that extend
to ~10,000 yr BP. The lakes include
three lakes that currently receive glacial
meltwater (Upper and Lower Kitandara
Lake, Lac du Speke) and four lakes that
do not (Mahoma, Upper Kachope, Batoda, Kopello). Comparisons between
these classes of lake allow us to disentangle the effects of climate-induced
vs. glacier-induced environmental perturbations.
Previous paleoclimatic research in
tropical mountains has been severely
constrained by uncertainty about the
relative influence of temperature and
precipitation on climate proxies. We are
Mountain Research Initiative Newsletter no. 4, May 2010
East African mountain lakes
New Afrotropical paleothermometers:
(a) Chironomid-reconstructed air temperature from 65 sites in East Africa against observed mean annual air temperature (Eggermont et
al., 2009a);
(b) MBT reconstructed air temperature from 65 sites in East Africa plotted against observed mean annual air temperature (Tierney et al.
submitted; Loomis et al. unpublished data)
developing novel, state-of-the-art paleoecological and organic geochemical
proxies for temperature using lake surface sediment measurements calibrated
against an extensive dataset of physical,
chemical, sedimentological and biological data on ~50 lakes and pools located
along an elevational gradient between
2500 and 5000 m asl (Eggermont et
al., 2007, 2009a-b; Russell et al., 2009).
Chironomidae (Insecta: Diptera; nonbiting midges) are powerful biological
indicators of past environmental change,
owing to their great taxonomic and
ecological diversity, short generation
times, efficient long-distance dispersal,
and good preservation of their larval remains in lake sediments (Walker 2001).
As a first step to develop chironomids
as paleothermometers in an African
context, we analysed their distribution
in the surface sediments of 65 lakes
and permanent pools in southwestern
Uganda (including virtually all lakes
on the Ugandan side of the Rwenzori
Mountains) and central and southern
Kenya (including a handful of lakes on
Mt Kenya) spanning a large mean annual temperature gradient (1.1-24.9°C)
(Eggermont and Verschuren, 2007; Eggermont et al., 2009a). The results (Figure 3a) highlight the great importance
of chironomids as paleothermometers,
but they likewise indicate that refine-
ment/elaboration with more Mt Kenya
lakes is desirable to increase model
performance. Furthermore, expansion
of the calibration dataset with other
regions (such as the Bale Mountains
in Ethiopia, and high elevation sites in
Tanzania) is also necessary to enhance
applicability of the chironomid-based
temperature models.
“Previous paleoclimatic research in tropical mountains
has been severely constrained
by uncertainty about the relative influence of temperature
and precipitation on climate
proxies.”
Organic geochemical methods have
provided fundamentally important
new insights into the precipitation and
temperature history of the continents.
For instance, application of the TEX86
temperature proxy, based upon distributional variations in aquatic Crenarcheotal membrane lipids, to sediment
cores from Africa’s great lakes has
documented temperature variations on
orbital and millennial time-scales over
the past 60,000 years (Powers et al.,
2005; Tierney et al., 2008). However,
Mountain Research Initiative Newsletter no. 4, May 2010
the application of TEX86 is limited to
large, low elevation lakes by ‘contamination’ from membrane lipids derived
from soil Crenarchaea. More recently,
variations in the relative abundance of
a new class of nine microbial membrane lipids, quantified by the so-called
MBT/CBT indices, have been found to
correlate to environmental temperature
and pH (Weijers et al., 2007). We have
investigated the distribution of these
compounds in our 65-lake dataset, and
found that these membrane lipids indeed
respond to and can be used to predict
temperature with considerable accuracy (Tierney et al., submitted; Loomis
et al. unpublished data). Questions still
remain regarding lipid source – an issue
that we are investigating through paired
analyses of soil, river, and lake sediment. Yet, this method clearly holds
promise in unravelling the temperature
history of East Africa’s Afroalpine environments.
Using short sediment cores taken at
the deepest part of the Rwenzori lakes,
we investigated changes in lacustrine
sedimentation, glacial extent, and biogeochemical processes in the Rwenzori
Mountains during the last 700 years by
comparing sedimentological (organic
and siliciclastic component) and or15
Long coring at Lac du Speke located at 4235 m in the Rwenzori Mountains (DR Congo) beneath the glaciers on western Mount Speke.
© Hilde Eggermont
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ganic geochemical profiles (carbon and
nitrogen abundance, ratio, and isotopic
composition of sedimentary organic
matter) from lakes occupying presently
glaciated catchments against profiles
from lakes located in catchments lacking glaciers (Russell et al., 2009). The
siliciclastic content of sediments in
‘glacial lakes’ significantly decreased
towards the present, whereas ‘nonglacial lakes’ generally showed weak
trends in their siliciclastic content over
time, demonstrating that changes in the
siliciclastic content of glacial lake sediments records fluctuations in glacier
extent.
ever, recession was underway by 1870
AD, during a regionally wet episode.
These data suggest that precipitation
variability does not appear to have been
the primary control on Rwenzori glaciers over long time-scales. Lastly, our
organic geochemical data indicate that
glacial retreat has significantly affected
carbon cycling in Afroalpine lakes, but
trends in aquatic ecosystem functioning are variable among lakes and require more detailed analysis.
Radiometric dating of our sediment
cores indicated that prior to their late
19th-century recession Rwenzori glaciers stood at expanded ‘Little Ice Age’
positions for several centuries under a
regionally dry climate regime. Evidence
for this drought includes lithostratigraphic data from crater lakes situated
immediately south east of the Rwenzori
(Russell et al., 2007; Bessems et al.
2008) and rising and high %Mg in calcite values from Lake Edward. How-
been the primary control on
„These data suggest that
precipitation variability
does not appear to have
Rwenzori glaciers over long
time-scales.“
We also evaluated the limnological
and ecological sensitivity of Rwenzori
mountain lakes to climate change by
comparing the species assemblage of
larval chironomid remains recovered
from sediments deposited recently with
fossil assemblages recovered from the
base of the short sediment cores which
are dated to within or briefly after the
Little Ice Age (Eggermont et al., 2010).
Application of the chironomid-based
temperature models indicated significantly warmer mean annual air temperatures (on average +0.38 ± 0.11 °C) at
present compared to between ~85 and
~645 years ago. Inferred temperature
changes are independent of whether
lakes are located in glaciated or nonglaciated catchments, and of the age of
the core base, suggesting that at least
part of the signal is due to relatively
recent, anthropogenic warming. The
fairly uniform and marked historical
warming trend in Rwenzori lakes documented by this study highlights their
ecological vulnerability and their value
as early-warning systems for detecting
the limnological and ecological effects
of global warming.
Future fieldwork will focus on the
expansion of the calibration data sets
with more lakes from Mt Kenya, and
recovery of long sediment cores from
Mt Kenya lakes. Together with ongoing multiproxy work on the long cores
Mountain Research Initiative Newsletter no. 4, May 2010
East African mountain lakes
from Rwenzori, these data will allow
us to construct a regional picture of African paleoclimate history and glacier
dynamics over the past 5000-10,000
years.
In addition, we aim to disentangle the
relative importance of local, regional
and temporal processes driving the
community ecology of Afroalpine
Cladocera (water fleas; Crustacea).
The study comprises various spatial
and temporal scales in the isolated islands of East Africa’s highest mountain
ranges, and integrates traditional techniques (downcore fossil analysis, DNA
barcoding) with more novel approaches
such as palaeogenetics. Besides providing fundamental insight into evolution-
ary and ecological drivers of multiscale community ecology, this project
will provide a means to assess global
change effects on the biodiversity and
ecological integrity of tropical cold-water lakes. As such, theoretical ecological concepts will eventually feed back
to bioconservation policy.
Authors & Principal Investigators
Hilde Eggermont, Limnology Unit, Ghent University, K.L. Ledeganckstraat 35, B-9000 Ghent, Belgium;
Royal Belgian Institute of Natural Sciences, Freshwater Biology, Vautierstraat 29, 1000 Brussels, Belgium;
[email protected]
James Russell, Geological Sciences, Brown University, Box 1846, Providence, RI 02912, USA;
[email protected]
Dirk Verschuren, Limnology Unit, Ghent University, K.L. Ledeganckstraat 35, B-9000 Ghent, Belgium;
[email protected]
Project participants
Shannon Loomis, Geological Sciences, Brown University, Box 1846, Providence, RI 02912, USA
Leen Audenaert, Limnology Unit, Ghent University, K.L. Ledeganckstraat 35, B-9000 Ghent, Belgium
Bob Nakileza, Mountain Research Center, Makerere University, Uganda
Daniel Olago, Chiromo Campus, Riverside Drive, P. O. Box 30197, Nairobi, Kenya
Stephen Rucina, National Museums of Kenya, Department of Palynology and Palaeobotany, Nairobi, Kenya
Geoffrey Mibei, Chiromo Campus, Riverside Drive, P. O. Box 30197, Nairobi, Kenya
Erustus Kanga, Kenya Wildlife Service, Langata Road, Nairobi, Kenya
Anouska Kinahan, Technical Advisor Frankfurt Zoological Society –Bale Mountains Conservation Project, Robe, Bale,
Ethiopia
Weblinks
Rwenzori Project website:
http://www.geo.brown.edu/georesearch/esh/Rwenzori.htm
Personal webpages:
http://www.ecology.ugent.be/limno/HE.php
http://research.brown.edu/myresearch/James_Russell
Mountain Research Initiative Newsletter no. 4, May 2010
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References
Bessems, I., Verschuren, D., Russell, J.M., Hus, J., Cumming, B., 2008. Paleolimnological evidence for widespread late-18th
century drought across equatorial East Africa. Palaeoecology, Palaeoclimatology, Palaeoecology 259: 107-120.
Eggermont, H., Heiri, O., Russell, J., Vuille, M., Audenaert, L., and Verschuren, D. 2009a. Chironomidae (Insecta: Diptera) as
paleothermometers in the African tropics. Journal of Paleolimnology DOI 10.1007/s10933-009-9339-2.
Eggermont, H., Russell, J., Schettler, G., Van Damme, K., Bessems, I., and Verschuren, D. 2007. Physical and chemical limnology of alpine lakes and pools in the Rwenzori Mountains (Uganda-Congo). Hydrobiologia 592: 151-173
Eggermont, H., Van Damme, K., and Russell, J.M. 2009b. Rwenzori Mountains (Mountains of the Moon): headwaters of the
White Nile. H.J. Dumont (ed). The Nile: Origin, Environments, Limnology and Human Use, pp. 243-261. Monographiae Biologicae, Springer Science.
Eggermont, H., and Verschuren, D. 2007. Taxonomy and diversity of Afroalpine Chironomidae (Insecta: Diptera) on Mount
Kenya and the Ruwenzori Mountains, East Africa. Journal of Biogeography 34: 69-89.
Eggermont, H., et al (2010). Ecological and hydrological sensitivity of Rwenzori Mountain lakes to climate warming. In H. Eggermont, M. Kernan, K. Martens (Eds). Global change impacts on mountain lakes. Special issue Hydrobiologia 648: 123-142
Powers, L.A., Johnson, T.C., Werne, J.P., Castañeda, I.S., Hopmans, E.C., Sinninghe Damsté, J.S. and Schouten, S. 2005. Large
temperature variability in the southern African tropics since the Last Glacial Maximum. Geophysical Research Letters 32:
doi:10.1029/2004GL022014.
Russell, J.M., H. Eggermont & D. Verschuren, 2007. Spatial complexity of Little Ice Age Climate in East Africa: sedimentary
records from two crater lake basins in western Uganda. Holocene 17: 183-193
Russell, J.M., H. Eggermont, R. Taylor & D. Verschuren, 2009. Paleolimnological Records of Recent Glacial Recession in the
Rwenzori Mountains, Uganda-D. R. Congo. Journal of Paleolimnology 41: 253-271
Russell, J.M. & T.C. Johnson, 2007. Little Ice Age drought in Equatorial Africa: ITCZ Migrations and ENSO variability. Geology 35: 21-24.
Tierney, J.E., Russell, J.M., Huang, Y., Sinninghe Damsté, J.S., Hopmans, E.C. and Cohen, A.S., 2008. Northern hemisphere
controls on tropical Southeast African climate during the past 60,000 years. Science 322: 252-255.
Tierney, J.E., Russell, J.M., Eggermont, H., Hopmans, E.C., Sinninghe-Damsté, J.S., and Verschuren, D. submitted. Environmental controls on branched tetraether lipid distributions in tropical East African lake sediments: a new lacustrine paleothermometer. Geochimica et cosmochimica Acta.
Walker, I.R., 2001. Midges: Chironomidae and related Diptera. In: Smol J.P., Birks H.J.B., Last W.M. (eds) Tracking Environmental Change Using Lake Sediments. Zoological Indicators. Kluwer Academic, Dordrecht, pp 43-66.
Weijers, J.W.H., Schouten, S., van den Donker, J.C., Hopmans, E.C. and Sinninghe Damsté, J.S., 2007. Environmental controls
on bacterial tetraether membrane lipid distribution in soils. Geochimica et Cosmochimica Acta 71: 703-713.
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Mountain Research Initiative Newsletter no. 4, May 2010